How Biomass Electricity Is Counted in Carbon Accounting
Background briefing for the Week 3 assumption audit exercise.
The rule
Under international carbon accounting rules (the UNFCCC framework), CO₂ released when biomass is burned for electricity is counted as zero.
This is not a measurement. It is an accounting convention. The chimney at a biomass power station releases real CO₂ — roughly 330 kg per MWh, comparable to coal. But the accounting rules assign this a value of zero.
The reasoning
The logic behind the rule is a carbon cycle argument:
- Trees absorb CO₂ from the atmosphere as they grow.
- When the wood is burned, that CO₂ is released back.
- New trees grow in their place and reabsorb the CO₂.
- Over a full cycle, the net emission is zero.
Therefore — the argument goes — biomass is carbon-neutral, and the emissions from burning it do not need to be counted in national greenhouse gas inventories.
Where the rule applies
The UK follows this framework. Biomass electricity (mainly wood pellets burned at Drax power station in North Yorkshire) is classified as renewable and zero-carbon in official energy statistics.
In 2024, bioenergy accounted for roughly 40 TWh of UK electricity generation — about 14% of the total. Under the accounting rules, none of the CO₂ from burning this fuel appears in the UK’s reported emissions.
What the accounting includes and excludes
| Included in “zero” | Not included in “zero” |
|---|---|
| CO₂ absorbed by trees during growth | Time delay before regrowth absorbs the CO₂ |
| CO₂ released at the power station chimney | CO₂ from harvesting, chipping, and drying the wood |
| CO₂ from shipping pellets across the Atlantic | |
| CO₂ from processing raw wood into pellets | |
| Changes to the forest’s ability to absorb carbon if it is not replanted | |
| Differences between fast-growing plantations and natural forest |
Key numbers
| Stage | Approximate CO₂ (kg per MWh) |
|---|---|
| Combustion (what the chimney releases) | 330 |
| Harvesting + processing + drying | 30–50 |
| Ocean shipping (US East Coast → UK) | 15–25 |
| Total lifecycle estimate | 390–400 |
| Official accounting value | 0 |
For comparison: coal ≈ 910 kg/MWh; gas ≈ 360 kg/MWh; wind ≈ 11 kg/MWh (lifecycle).
The carbon payback question
Even if new trees are planted, they take decades to reabsorb the CO₂ released at combustion. Estimates of this “payback period” vary widely:
- Managed fast-growing plantation (SE United States): 20–40 years
- Natural hardwood forest: 40–100+ years
- If the forest is not replanted: payback is never achieved
The UK’s net zero target is 2050 — roughly 25 years away. If the carbon payback takes 40–100 years, biomass burned today will not be carbon-neutral within the policy-relevant timescale.
Your task
Work in pairs. Using this summary:
List the assumptions the “zero at combustion” accounting rule makes. (Aim for at least five.)
For each assumption, ask: Is this testable? What evidence would challenge it?
Which assumptions, if wrong, would most change the conclusion? Rank your top three.
You will use these answers in the application session, where you’ll test what happens when the assumptions change.